A method for recording on an optical recording medium by holography generally utilizes optical interference between an image information-carrying information beam (object beam) and a reference beam inside the medium to produce an interference image (interference fringes), and records it in the medium.
Holographic optical recording that involves application of such information and reference beams with their optical axes matched is referred to as “collinear holography”.
Application of information and reference beams by collinear holography produces interference fringes in the recording layer of an optical recording medium, thereby recording information therein. Reproduction of the information recorded in the optical recording medium is achieved by application of a reproduction beam, which is identical to the reference beam used for recording, from the same direction as the reference beam. This leads to generation of a diffracted beam from the interference image formed of interference fringes as optical information, and the optical information is reproduced by collecting the diffracted beam.
As a method for increasing the storage capacity of optical recording media, there are proposed multiplex recording methods for increasing the density of the interference fringes recorded. More specifically, shift-multiplex recording, angle-multiplex recording, wavelength-multiplex recording, and phase-multiplex recording have been utilized.
Among these recording strategies, the shift-multiplex recording has high compatibility with conventional CD and DVD recording methods where information is recorded on a spinning disc, and offers high random access performance. This is because in this recording method, additional information, or interference image, is placed one after another on the initial recorded information while slightly moving either a beam spot or an optical recording medium along the surface of the recording layer. For this reason, shift-multiplex recording is employed for the above-mentioned collinear holography where single lens is used for recording (see Non-Patent Literature 1).
As shown in FIG. 4, shift-multiplex recording by collinear holography is carried out on a recording pulse beam basis, i.e., on a recording beam spot basis, which beam spot has a predetermined size and is produced by focusing an information beam and reference beam 39, which have been passed through an object lens 12, in a recording layer of an optical recording medium 21. When a beam spot has been applied to a recording area and information is recorded, the optical recording medium 21 rotates by one pitch of a distance of p in a counterclockwise direction which is opposite to the direction indicated by the arrow in FIG. 4. In this way a beam spot is applied for every one-pitch rotation to the next recording area, whereby recorded information is sequentially multiplexed.
In the shift-multiplex recording, when the recording pulse is used, a portion where the information beam and reference beam are focused (a recording spot formed in a substantially circular shape with a spot diameter of about 200 μm) contains information of approximately 30,000 bits (about 4,000 bites), and the information is recorded as one page, and information is recorded on a data page basis (see Patent Literature 1).
The reason why such a large volume of information of approximately 30,000 bits is recorded in one data page is to increase the recording density.
In order to record such a large volume of information, for example, as shown in FIG. 1, a recording pulse 26 is used once every approximately 200 μsec with a peak intensity of light of 100 W for 30 nsec as irradiation time.
However, in order to record such a large volume of information in a short period of time, like 30 nsec of the light irradiation time, 15 mW is required as the average energy amount. As an average energy density required for recording in the recording spot of having a diameter of about 200 μm, about 9.5 mW/cm2 is required. Therefore, as a total recording energy, energy of about 1 mJ/cm2 is necessary.
However, in order to record information with a high energy of about 1 mJ/cm2 in a short period of light irradiation time of 30 nsec, it is necessary to prepare a light source having a relatively high-capacity, a large size spatial light modulator (SLM) for processing a large volume of information of approximately 30,000 bits, as well as a photodetector composed of CMOS and the like, which makes it difficult to use optical recording method by holography to small-size optical systems such as optical systems used for present CD and/or DVD.
An optical recording method that allows for downsizing of optical systems by reducing the irradiation energy per one recording during irradiation of an information beam and a reference beam as well as allows for application of holographic recording to small-size optical systems used for CD and/or DVD, and an optical recording apparatus and an optical recording medium that can achieve the purposes have not yet been provided so far, and it has been desired to provide the method, apparatus and optical recording medium.
[Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2004-177958
[Non-Patent Literature 1] Nikkei Electronics, pp. 105-114, Jan. 17, 2005